1. Field of the Invention:
The invention relates to a method and apparatus for controlling the magnitude of an analog current through a load impedance to be proportional to the magnitude of a voltage input signal.
2. Description of the Prior Art:
An analog current loop output circuit is typically used to monitor a process parameter such as a temperature or pressure in a controlled process. A conventional analog current loop output circuit includes a load impedance which may be, for example, the impedance of a strip chart recorder for recording a particular parameter. A power supply hving a fixed voltage drives a current through the load impedance. A linear current modulator is connected in the loop for controlling the magnitude of the current in the loop to be proportional to the magnitude of an input voltage which corresponds to the measured parameter. In its simplest form the linear current modulator may comprise a variable resistance which is controlled by the input voltage so that the current in the loop varies with the input voltage. Such conventional analog current loops are inherently inefficient because the load impedance dissipates 50% or less of the total power furnished by the power supply, with the remaining power being dissipated by the current modulator.